


The Long Haul

by wearyeyebrow



Category: Undertail - Fandom, Undertale (Video Game)
Genre: Blow Jobs, Cunnilingus, Demisexual Sans (Undertale), Ecto-Penis (Undertale), Established Relationship, Explicit Sex, F/M, Fluff, Hand Jobs, Insomnia, Knotting, L-Bombs, Masturbation, Mating Cycles/In Heat, Profanity, Sensitive bones, Slice of Life, Vitamin B12
Language: English
Status: Completed
Published: 2017-08-08
Updated: 2017-08-08
Packaged: 2018-12-12 13:51:06
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 5,348
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/11738382
Author URL: https://archiveofourown.org/users/wearyeyebrow/pseuds/wearyeyebrow
Summary: The heart wants what the heart wants. Apparently, so does the body. It was bound to happen sooner or later. The question is: What will Sans do about it?My goal was to make an obligatory 'heat' fic that's as realistic as possible - but let's face it, we can only do so much with magic skeletons. I can't promise perfection, but I can promise meaningful conversation peppered among explicit sex scenes.I hope this is as fun to read as it was to write. Enjoy!





	The Long Haul

**Author's Note:**

> I decided to throw caution to the wind and create my own version of 'being in heat.' A combined cocktail of traits that features knotting, because it’s one of my favorite things, and I need an excuse to incorporate it in monster sex/biology. Hopefully you’ll get the gist. Thanks for giving it a read!

At first, it was subtle. His lined jackets suddenly too scratchy to wear. Or the smell of the garbage prompting him to take it out. A chore he’s never done because it’s, well, a chore. And he has a reputation to maintain.

Now, it isn't so subtle. He’s wired, and exhausted, trying to think through his emotions. You are someone he’s known for over ten years. In the three years you’ve been seeing each other, not once have you ever felt like someone he needed to check on, or worry about. You’re perfectly capable and independent. He appreciates that about you and your relationship. You each have your own thing. You don’t need to be around each other 24/7 to feel validated.

At least he didn’t feel that way three days ago. It crept up on him.

It’s five o’clock in the morning, and he’s lying in a pool of tangled sheets, staring with blank eye sockets at the white expanse of his ceiling. He can’t sleep.

What are you doing right now? You should be asleep. He hopes you’re asleep. You might have stayed up all night, working on a project, carrying a team of projects. You might get sick if you don’t get enough sleep. That’s a thing, isn’t it? Humans need sleep to regulate their immune system.

What about diet? He’s never considered that. Humans have very specific nutritional needs… oh for fucks sake. You’re in your thirties. You know how to take care of yourself.

…But what if you need more vitamin B12?

He groans, dragging a hand across his face. Think of something else, something else. He flexes his toes and uncomfortably shifts. His attention is drawn to the subtle ache at his pelvis, reminding him why he had been thinking about you in the first place.

Maybe he can go with it, just for a bit. He gives in and throws an arm over his eye sockets.

Thoughts flit in and out of his mind, ideas and images flashing by. What are you doing right now? He had been wondering. Maybe you’re just getting to sleep. Maybe it’s been a long night, and you’re tired, but not ready to go to sleep yet. Maybe you’re looking for something to do, some way to relax. Maybe you…

Maybe you’re in bed, the corner lamp still on. Maybe you’re on your back, spine arched as you stretch, yawning. Maybe you… maybe you lazily run a hand over your shirt, gliding over your breasts, thinking about whether it’s worth it.

He breathes out harshly through his nasal cavity.

Then you take off your shirt, throwing it somewhere on the floor. You brush your fingertips over your nipples, sighing as you lean back on one hand. You lie down, arching again…

He carefully traces the very bottom of his rib cage, the tip of his sternum, shirt hiked up.

Maybe you… maybe you kick off your shorts and slide a hand down your stomach, over your underwear. Maybe you buck your hips. Fuck.

His own hand shifts lower, thumb brushing the top of his hipbone.

Maybe you arch your neck, and moan softly, teasing yourself. You pull your underwear to the side and rub a finger against your entrance. A thin, translucent mucus connects your finger to your cunt as you pull away. You chuckle to yourself, eyes hooded.

Then maybe you fall back onto the pillow and slide your underwear off. You breathe harshly as you circle your clit, letting out adorable, high pitched sounds. Those are his favorite when he eats you out.

He can almost taste you on his tongue, magic threatening to form in his mouth.

Your hand on his head as you fuck his face, praising him, telling him what a good job he’s doing… how much you – ‘fuck,’ he whispers, imagining himself gripping your thighs. The way you tense when you’re about to come, gasping. The way you kiss him just after you’ve finished, tasting yourself on his tongue-

He clenches his teeth, eye sockets squeezed shut. Calm down. Go back to… back to earlier, back in her bedroom, having some time to herself. The intense feeling of surging magic simmers down.

That’s right you were, you were about to… Maybe you reach beneath your bed and snag a toy. It’s a relatively small dildo, silicone, matte finish. You’ve used it on him many times.

His toes curl.

You shrug and spit on it, not bothering with lube. You lean back and spread your legs, eyes closing as you slowly slide it in to the hilt.

You circle your clit with one hand, and start fucking yourself with the other. Slowly, sighing at the sensation. Then you gasp as you hit the right spot, whining and throwing your head to the side. You strain to watch the dildo move in and out, faster and faster, bucking your hips.

He can feel magic center at his pelvis as he taps on his pubic symphysis, squirming for more, but refusing himself.

Maybe you start whispering, a string of expletives and words of affirmation. You could be fantasizing about anyone, he doesn’t care. But you’ve confessed to whispering his name in the middle of the night, just like he whispers yours.

You’ve also admitted to whispering Toriel’s name, but then again so has he - your mutual crush a well-known, inside joke.

You moan his name, gasping, shaking, about to come, and fuck he wants to see it. He rubs the underside of his pubic symphysis, feeling the ache set in. Fuck he wants to come. Can he come without getting too excited? Is the question.

Finally, you freeze. Body tightening as you moan into your pillow, still feebly fucking yourself.

He moves his phalanges faster, breath hitching.

Then there you are in his mind perched above him, as you so often are, a completely different scene. You bite his neck and he can’t help tilting his head to the side, trapping sound behind his teeth.

Maybe you run your hands up his body, taking advantage of how well you know him. You whisper words in his ear that run together, filling up his soul, heating up his body. He’s close. He rubs faster, voice choking with effort to keep himself quiet.

But then he feels the swell, hears the beginnings of fusing magic and he immediately, regretfully, retracts his hand. He grits his teeth.

Fuck, fuck, fuck. He can’t. He can’t let it get that far. His brother rarely sleeps, only shutting down for a few hours each night in an attempt to ‘mimic the human experience.’ Those couple of hours happen to be now, and he can hear soft snores perforating the thin wall that separates their bedrooms.

Any other time he doubts he’d let that stop him. He’s a master at keeping quiet after years of sharing a wall with his baby brother.

But there are too many unknown variables, and he isn’t brave enough to test it. He clutches the sheets in frustration, breathing heavily. A bead of sweat slides down his forehead.

What is he doing? He could be with you right now. You’ve already talked about it; he wasn’t about to leave you in the dark on this one. You talked about monster and human biological differences for years during your friendship. A couple years into your romantic relationship, and this shouldn’t come as a surprise. Rather, it’s an expected, natural progression.

Looking down at his femurs, magic sparks, fizzles. It contracts, lighting his bones on fire, making him weak-kneed.

He falls back onto his mattress and glances at the clock. It’s a little after six, which means you’ve gotten up but you haven’t left for work yet. He breathes out slowly and waivers for a bit before grabbing his phone, momentarily blinded by the screen, and calls you before he can change his mind.

The phone rings twice before you pick up.

‘Good morning!’ You say, entirely too awake for 6:00 am.

‘heh, morning.’

‘What’s up?’ He can hear the squeaky sound of a faucet being turned in the background. Static roars as your shower surges to life.

‘i know it’s early, but is it cool if i come over?’

‘Sure. I won’t be here for that much longer, but you have a key - chill for as long as you want.’

‘thanks. see you in a bit.’

‘See you.’ The phone clicks, and the buzzing stops.

He chucks on some clothes and writes a hasty note to his brother, locking his bedroom door before he blips out of existence.

-

Huh. Sans up this early? Is his house on fire? You briefly consider this option, remembering Undyne’s tragic tale.

Nah. Maybe Papyrus tried to get him to exercise again and he’s hiding at your place. That’s more likely.

You step into the shower, and not more than a few seconds later you hear an elaborate, marimba like knock on the door, letting you know he’s here.

You finish fifteen minutes later and turn the shower off, the handle squeaking like old brake pads. You blow dry your hair until it’s gone from soaking to damp and decide that’s good enough.

You shiver at the temperature change as you open the door, condensation still clinging to your skin, wrapped in a towel.

He’s sitting on the edge of your bed, dark circles underneath his eye sockets. He looks up at you.

‘hey.’

‘Hay’s for horses.’

‘that’s the most dad-like thing you’ve ever said.’

‘Of course. I’d make a great daddy.’ The innuendo is not lost on him and he snickers.

‘not sure if i’m into that, but i’ll try almost anything once.’

‘You were sure to include that ‘almost.’

‘can’t be too careful with legal transactions.’

‘Okay, now you’ve lost me.’

‘yeah.’

Usually he’d have a quip ready to go, but you know he isn’t a morning person.

You walk over to your closet and flick through the hangers, trying to find something comfortable to wear.

‘I’m surprised you’re awake. Papyrus get you up again?’

‘not exactly.’

You catch him staring in your peripheral vision. ‘Like what you see?’ You laugh.

‘uh, well, yeah?’

‘I try.’

He chuckles, but his laughter feels strained, like something’s on his mind. You turn back to your closet. ‘I guess you don’t have to have a reason, but why are you up so early?’

He breathes out slowly, ‘kind of need to talk to you, if that’s cool. i know you’re getting ready for work.’

‘I can get ready and talk. What’s up?’

You’re met with silence. You look at him quizzically. ‘Is everything okay?’

‘i mean yeah, technically.’ He finally says.

You forgo getting ready and sit down next to him. ‘What does that mean?’

‘i’m making this a bigger deal than it has to be, it’s just hard to bring it up, and it shouldn’t be.’ He sighs. ‘look, we’ve talked about both human and monster biology, yeah?’

‘Yeah.’

‘then we’ve talked about monster reproduction.’

‘Sure, we’ve talked about it.’ Your gaze softens from confused to curious.

‘you don’t have to do anything about it, just putting that up front. but, i’m uh, i’m in heat. and i figured that it kind of concerns you too, just a bit.’ He laughs nervously, rubbing the back of his neck. He breathes out, and sighs, ‘there it is.’

‘Hmm.’ You sit and think for a minute. ‘You know… I’ve done my own research too. I mean, we both have. You know more about us than the average human, and I’m no slacker. But I'll listen to you before I quote research. Just... just talk to me. It's just me.’

‘okay.’ He leans to the side and rests his forehead against your shoulder, his posture shifting from rigid to slouched – much more like himself. 

‘I can take a vacation day. Lord knows I have plenty of them saved up.’

‘i won’t ask you to, but i'm not gonna protest if you do.’

‘Hm.’ You get up, snag your phone from the bathroom, and dial your boss. A few moments later and you get the okay to take the day off. It’s a Friday, so work’ll be slow.

‘We’re good.’

He doesn’t want to admit how relieved he feels.

‘How long has this been going on?’ You ask, eyebrows furrowed. ‘Not like you had to tell me about it, but…’

‘didn’t really put the pieces together until a couple days ago. and even then, it hasn't been around for much longer than that. just didn’t seem like a big deal.’

‘That changed?’

‘last night.’

‘Didn’t sleep so well, huh?’

‘that’d be a big no.’

‘Is it painful?’

‘no, nothing like that. you’ve one-upped us in the pain department.’

‘That’s not an award I want to have.’

‘don’t blame you.’ He closes his eye sockets. ‘but it is a little difficult to deal with.’

‘In what ways?’

He sighs, flopping back on your bed, staring at your equally white expanse of ceiling. ‘like… you know when a singer adds one too many riffs in a song? it feels like that. like everything is too much. good things feel great, and bad things feel horrible. like polar fleece.’ He shivers.

‘Or velvet.’

‘especially velvet.’

You drop your towel and throw on some shorts and a t-shirt, fully aware they won’t be on for long.

‘What else?’ You probe.

‘that’s about it really, up until the past few days.’

‘Past few days?’ You sit on the edge of the bed, weight resting on one arm.

‘the anxiety,’ he murmurs. ‘not that i don’t like thinking about you, but you are a weird constant. and i keep finding things to worry about. things that aren’t worth worrying about, things that don’t make sense. and then… sex is great, but i’ve never felt like i needed it.’

‘Until now?’

He nods. ‘s’weird, right?’

‘Nah.’

‘should be, from your perspective.’

You lean over him, hair framing your face. ‘Nah.’ You kiss his cheekbone. ‘So, knotting?’

He chuckles, clearing his throat. ‘yeah. s’where the excess magic for reproduction comes in. get it? comes in. if you were a monster, we’d need contraception.’

‘Since this is the only time you guys can reproduce.’ He nods. ‘You ever gotten to this point with someone?’

‘well…’ he sits up and rubs the back of his neck, ‘it’s a magical response to emotional stimuli, and considering my penchant for casual relationships, you’re my first.’

‘What a coincidence, you’re mine too.’

‘heh, what are the odds?’

‘Astronomical.’ You cup his face in your hand and tilt his head back to kiss his teeth. His hands instinctively come to rest on your hips.

You pull away and he breathes out slowly. ‘i don’t want to hurt you. theoretically, it’ll be biggest right after i come, so just giving it a go seems like a bad idea.’

‘Not the most glamorous ER visit, huh? What if I wait till then? If I can get it in, we’ll be fine. I’ve done some size stuff before, so I’ll have a pretty good idea if I can get it to work or not.’

‘s’fine so long as it’s safe.’

‘if it doesn’t work, we’ll just do it differently.’

‘hasn’t stopped us before.’

‘Nope.’ You shift closer to him and kiss his jawline, a hand falling to his waist. He predictably tilts his neck, and you kiss his cervical vertebrae, tongue tracing along indents and divots, dipping into tiny grooves of bone. He shudders. ‘going... going in for the kill right away, huh?’

‘Uhuh.’

‘…thanks.’

‘For what?’

‘for doing this, taking the day off, you know.’

‘This isn’t a favor, Sans. I want you. Just like I always do.’ You kiss the space between his neck and shoulder.

‘so you’ve told me.’ He sighs softly, breath hitching as you start running your fingers across the bottom of his rib cage.

‘It’s true.’

‘i know.’

You push him onto his back, lips hovering over his teeth. ‘It’ll be easier with me on top.’

‘okay,’ he murmurs, eyelights searching your face.

‘Okay.’ You kiss the space between his eye socket and nasal cavity, and feel his hands come to rest at your hips. He stops breathing for a second, and you almost wish you could take a snapshot of this moment, as cheesy as that sounds.

He takes off his shirt and your fingers are featherlight between his bones. His hands find their way up your shirt. You shrug it off and drop it somewhere on the floor. Your breath hitches at his teeth on your neck, a hand on your breast, thumb on your nipple. You find his lumbar vertebrae, press against cartilage, and hear him groan into your skin.

He lifts his hips and you slide off his shorts. You wedge yours off and the hem gets caught on your foot. You both laugh breathlessly as you free yourself.

Your mouth is on his neck as you trace the outline of his pelvis, feeling the complexity of bone. You rub his pubic symphysis between your fingers. He grinds against your hand and you can feel magic gather, the sensation familiar and tingly.

He breathes in sharply as his cock materializes, already hard, with what looks like a partially swollen knot.

Your mouth waters as you kiss his shoulder. ‘C’mere. Sit on the edge of the bed.’ He does as he’s told.

You kneel in front of him and run your hands down his femurs. His breathing is shallow.

The first thing you do is run a finger over his knot. It’s hard and unyielding. It’ll take some work to get this inside you. You draw your tongue up his shaft, flicking against his frenulum. He makes a strangled sound above you, clutching the bedspread.

The noises he’s making have you dripping. You spread your legs and slip a finger inside of yourself. You make sure he’s watching. Then you lower your mouth onto his cock.

‘oh my god…’

You suck him off while you fuck yourself. You add a second finger.

He gasps above you. ‘You good, sweetheart?’

‘y-yeah, fuck i’ve wanted this all night – wanted you so fucking bad.’ He chokes on whispered words, propped up on his forearms as he watches you tease his knot with your tongue. He isn’t usually this forward, at least not without a little more work. Not that you mind.

You slip a third finger inside yourself, groaning as it almost doesn’t feel like enough. You need more. You want more. You take him in your mouth and suck, reveling in the taste of salty pre-come trickling down the back of your throat.

You feel his bony hand come to rest on your cheek. His thumb brushes hair out of your eyes. ‘i – i want…’

‘Hmph?’ You look up at him through your eyelashes and slowly pull off of his cock. Your breath is hot against him. ‘What do you want?’

‘let me…’ he gestures for you and you stand. His hands grasp the backs of your thighs, pulling you toward him. You see the soft, floaty magic of his tongue materializing in his mouth. He rests his forehead against your stomach. His fingers slowly trace your inner thighs.

‘lie down,’ he whispers heavily.

‘Gonna have to let go first,’ you breathe.

‘mm.’ He can’t seem to make up his mind. Instead of pulling away he pulls you down toward him, and you meet him half-way for a kiss.

Kissing him took some getting used to, but now you can't get enough. His tongue mixes with yours before he pulls away. ‘okay.’

In your haze it takes you a second to process what he’s saying okay to. He lets you go, and you circle the bed before lying on your back. You prop yourself up against some pillows. You know what he wants, and fuck, you’re pretty sure you want it even more than he does.

He crawls up the bed and settles between your legs, grasping your thighs as he inhales. 'love this,' he murmurs. He gently bites your inner thigh before nuzzling against your skin, 'love this...'

You gasp as he gets to work, tongue softly teasing your clit, running along your lips. You try not to dig your nails into his skull. ‘So… j-just like that.’ You can't help shifting your hips against his mouth. ‘Sans, baby boy, you feel s-so fucking good.’ He moans, the sound muffled by your cunt. His hips shift against the sheets.

‘Yesss…’ you whisper, your toes curling, 'You're perfect.' His breath hitches and he speeds up, eating you out with passion. It’s sweet torture, and you ache for something inside you.

You arch your neck as he speeds up, his tongue moving expertly, knowing just what you like. You love looking at him between your legs, the view alone has you close. You tap on his head and he stops, breathing heavily. His face is smeared with your cum. ‘you – you okay?’

‘J-just don’t wanna come yet, and if you keep doing that, I won’t be able to help myself.’

‘why is that,’ he pants, ‘a bad thing?’

‘I wanna wait this time, wait till you’re inside me.’ He groans, pressing his forehead against your thigh, and nods.

‘C’mere, switch with me.’ He wipes your cum from his mouth and then licks it off his fingers.

There isn’t even a need for lube with the amount of pre-cum dripping down his cock. You sit on his legs and wrap your thumb and forefinger around his knot, and your other hand around his shaft. You start moving, long languid motions, slow and torturous.

He arches his back and you can feel his legs strain beneath your weight, trying to move. He moans long and loud, hands clutching the pillow beneath him. He alternates between squeezing his eyes shut, and straining to look at what you’re doing.

‘That’s it…’ you can’t help murmuring, watching affectionately as you work him open.

You bend in two and take the tip of his cock in your mouth, tongue swirling around his slit. ‘fuck, fuck, fffuck, I’m, ah- i’m so close, ______, so close, so-‘ His eyelights roll into the back of his head, and his hands fist the sheets.

‘Wanna feel it, wanna feel you come,’ you mumble, harshly breathing through your nose.

It only takes a few seconds longer of him gasping, voice catching in his throat, before he throws his head back in a silent scream. His body freezes and you feel a rush of cum flood your mouth. His silence only lasts a moment, before he covers his mouth and moans loudly, practically sobbing. You can feel his cock pulse in your hand.

He whimpers, hips shaking, ‘h-holy fuck, fuck, fuck, fuck it’s not stopping. i knew but – it’s so,’ he covers his mouth and squeezes his eyes sockets shut, brow bone furrowed. 

You brush your hand over the one covering his mouth and he opens his eye sockets, eyelights bright and wild. You kiss the back of his hand, and then kiss him properly when he removes it. He moans loudly into your mouth, hand fisting your hair.

You pull back. 'M'gonna try okay?'

‘pleeease.’ He groans.

You kneel over him, and he jumps when you wrap your hand around his cock. You can feel cum drip down your hand as you guide him inside of you.

Other than the fact that he’s in the middle of an insane orgasm, it doesn’t feel much different.

His hand feebly brushes against your thigh.

‘Too much?’ You murmur.

‘n-no, better, a lot better.’ He tries to breathe deeply, calming himself a little.

You reach his knot and pause. It isn’t a gradual taper. It’s harsh and a little bit bigger than you expected.

You wiggle and twist your hips, trying to relax. You grind down and you feel a dull ache. You recognize the feeling of being stretched. He quivers underneath you, hands running up and down your hips in a soothing motion. You’ve gotten dildos about this big inside of you. But it’s been awhile, and the transition was never so daunting.

‘a-any time it hurts we’ll stop, okay?’ He stammers, trying his best to keep still. He fights through the fog and hazy pleasure of you grinding against him, of his cock twitching inside of you.

‘I’ll,’ you breathe out, ‘I’ll take this slow, and I’ll tell you if I can’t do it.’

He nods, squeezing his eye sockets shut.

He opens them at your laughter, soft and airy. You’re concentrating, lips parted, eyes clouded over and glassy. Your hair sticks to your neck, and tiny beads of sweat linger on your cheeks. Having you this close, but not being able to touch you properly, is maddening.

It takes a good few minutes of grinding and stretching on your part, and trying not to go cross-eyed on his. He comforts you as best he can, thumbs rubbing small circles into your thighs.

You roll your hips against his knot, the pressure builds, and then finally your body sucks him in. It’s a locking sensation, stuffing you more than you thought possible. You gasp and double over, curling your fingers around his ribcage, hanging on for dear life. There’s a mixture of pain and immense pleasure, the base of his knot providing constant pressure against your g-spot.

He whines in the back of his throat, breathing ragged, toes clenching, phalanges digging into your skin. ‘you – you okay, _____?’

‘Y-yeah,’ you stammer, ‘Just, just give me a minute.’

‘d-does it hurt?’

You shake your head, ‘No. It’s more like…’ you can’t help but chuckle, ‘It’s more like pressure. Like your dick is the cork in my wine bottle.’

Despite everything he laughs, body shaking, hips shifting slightly. You gingerly move to reposition yourself, finally seated all the way, weight resting on his body.

You squeeze around him and your clit throbs. The foreign feeling starts to melt away, and all you feel is mounting pleasure. ‘F-fuck Sans, it’s…’ you moan softly as you twist your body, ‘this feels fffucking incredible.’ Your voice shakes, and you feel like you’re losing your mind. ‘T-took long enough, but holy shit, what the fuck, I’m not even moving.’

Your words bring a delicious flush to his cheekbones. For just awhile this is the only thing that matters, you are the only thing that matters. And he’s not phased. He’s not overthinking. He’s just lost in this feeling.

You start rocking your hips against his body, his knot anchoring you. ‘Oh my god,’ you breathe in. The sensation is overwhelming. He gasps, eyelights rolling in the back of his head. His hands are on your back, phalanges digging into your skin.

‘yes, yes, yes, yes,’ he moans, and a small bit of drool drips down his chin. You bury your head in his shoulder and groan at the feeling of his warm cum filling you up. You reach one hand toward your clit, rubbing your fingertip in tight little circles. It feels incredible, like nothing you’ve ever experienced, and holy fuck - your orgasm hits unexpectedly. You cry out and clench around him.

‘you comin’? you’re… fuck, yes, god you feeeeel, you, ah,’ he can’t form sentences, his mind turned to mush.

You spasm around his cock, walls tightening painfully around his knot. You mumble into his shoulder, hips twitching sporadically. You feel his hand on your back, softly gliding over you as he practically growls beneath you.

‘______, can we - i need to move.’ He breathes.

‘What do you-’

‘can i – can we switch? just places, just…’

You laugh a little, breathless as you gingerly switch positions, you still tied with him, until your head hits the pillow and he’s above you.

He’s breathing heavily, hands planted on either side of you. His body shakes, toes curling, but he doesn’t move, forehead pressed into your clavicle. You wrap your legs around his pelvis. You touch your hand to his cheekbone, and he tilts to kiss your palm. He breathes in sharply and groans as he starts to move, slowly, the same pace you were forced to take. He buries his face in your neck, and moves with what little leverage he has, his knot keeping you tied to him.

‘So good,’ you moan, wrapping your arms around him, clutching the back of his rib cage.

He speeds up at your obvious pleasure, and you reach down to rub your clit.

You can feel it this time, mounting pleasure, the build incredibly fast. You arch your back and squeak his name through your teeth.

You feel another rush of cum. Every sound he makes is muffled by your skin. But you can hear him, right below your ear. You can feel his ragged breath against your neck, his sweat on your skin, and you can smell his arousal. Intoxicating, inviting.

He sobs as more cum spills inside of you. You hear your name, and you tilt your head to kiss his skull. ‘Sans,’ you murmur, ‘Fill me up.’

You lose track of time, you lose track of how many orgasms you have, your mind is vacant, your words are sweet in his ear. His body shakes, magic almost completely drained. Eventually, his hips slow, his cum a steady, weak stream. You can feel his knot begin to shrink.

You praise him in your delirium and he mumbles into your neck.

His entire body shakes with exhaustion and your cunt is sore. With effort he lifts himself off of you and holds your hips as he pulls out. You shiver when you feel a rush of cum spill out of you. His eye sockets are half-lidded as he stares, phalange lightly brushing over your exposed clit. Your breath hitches.

‘if-if i lie down i’m gonna fall asleep. you wanna take a shower first?’

‘Yeah,’ you rasp, ‘Probably a good idea.’ You sit up and stumble as you try to stand. He catches your arm and steadies you. ‘you okay?’

‘Aches a little, but nothing I can’t handle. Was worth it.’ You chuckle, leaning against his shoulder, suddenly exhausted. You guide each other to the bathroom, cum sliding down your legs, and splattered against his. He starts the shower and feebly tries to help you wash, half-asleep on his feet, but you appreciate the gesture.

‘How often does this happen?’ You murmur under the water.

He yawns, ‘depends. usually a yearly thing, but i’ve heard it’s unpredictable.’

You get out of the shower and sit on the toilet wrapped in a towel. He dries off your hair as your eyelids droop.

You get back to bed and rip off the sheets, throwing them in the direction of your dirty clothes hamper. You briefly consider new ones before face planting onto the bed, deciding it isn’t worth it. He grabs a blanket from the closet.

You roll over to face him as he sits on the edge.

‘_____?’

‘Hmm?’ He’s silent for a moment, and you fight through your exhaustion. ‘You okay?’

He hands you the blanket. ‘...means a lot, that you’d do this with me. it’s… well, you know that it’s a pretty big deal for us.’

You fight back a yawn and curl your body around his waist, blanket pooling at your naval. ‘That’s exactly why I wanted to do it. We’re in this for the long haul, so I hoped it’d happen at some point. M’glad that it did.’

‘yeah,’ he murmurs, ‘me too.’

You fight to keep your eyes open. ‘Sorry Sans, I can’t… I’m fading fast here.’ You scooch back and open your arms, eyelids heavy. He falls into them and you tangle your legs together. ‘Let's take a nap.’ You mumble.

‘would i ever say no to that?’

‘I’d be worried if you,’ you yawn, ‘if you did…’

His forehead falls against your clavicle, hand coming to rest in the space between your breasts, feeling the light outline of bone beneath your skin.

‘love you.’ he murmurs, almost too quiet to hear.

But you hear him anyway.

‘Love you too.’

**Author's Note:**

> I had a lot of fun writing this, and learned a lot about estrous cycles (said knowledge not applied). Who said fanfiction couldn't be educational? Thanks for reading!
> 
> And as always, constructive praise and criticism are greatly appreciated.


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